Transmission of guided waves



Apnl 1940' s. A. SCHELKUNOFF TRANSMISSION OF GUIDED WAVES Filed Sept. 4,1937 FIG.

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FIG. 3

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lNVENTOR 5. A. SCHEL KUNOFF A T TORNE Y Patented Apr. 30, 1940 UNITEDSTATES TRANSMISSION OF GUIDED WAVES Sergei Alexander Schelkunoff, NewYork, N. Y.,

assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y.,a corporation of New York Application September 4, 1937, Serial No.162,422

16 Claims.

This invention relates to high frequency electromagnetic wavetransmission systems and more particularly to the guided transmission ofhyperfrequency electromagnetic waves.

In my copending application Serial No. 56,959, filed December 31, 1935,which issued on February 21, 1939, as U. S. Patent No. 2,147,717, it isshown that electromagnetic Waves of certain kinds can be guided througha metallic pipe containing only a dielectric medium. Guided transmissionof this kind is possible only at frequencies exceeding a critical orabsolute cut-off frequency that is dependent on the transversedimensions of the guiding structure, the guide thus presenting thecharacteristics of a high-pass filter. There are many types of suchwaves, each distinguished by its characteristic field pattern, and foreach type there is a corresponding critical frequency, generally higherthan the absolute cut-off frequency, which must be exceeded if a wave ofthat particular type is to be propagated within the guide. There is onlyone type of wave that can be propagated within the guide at frequenciesso low as to approximate the absolute cut-off frequency, but atfrequencies above the next higher critical frequency, not only a wave ofthis type but also one or more independent Waves of different type,travelling at diiferent velocities, will originate at the transmitterunless special precautions are taken to prevent it.

A principal object of the present invention is to provide a guided wavesystem of the kind described in which the absolute cut-oil frequency, orminimum frequency of transmission, is substantially lowered withoutincreasing the overall dimensions of the guide. Another object is toincrease the frequency range over which the guide will transmit but asingle type of guided wave.

The present invention in one of its simplest embodiments comprises ahollow copper tube having a longitudinal copper fin or baflle extendingradially inward from the periphery of the tube, together with means forlaunching electromagnetic waves of suitable character in the tube. Theforegoing embodiment is typical and illustrative only, however, and thenature of the present invention and its various objects, features andadvantages will appear more fully in the detailed description that is tofollow. Reference will be made to the accompanying drawing, in which:

Figs. 1 and 2 illustrate typical transmission systems embodying thepresent invention;

Figs. 3 to 11 represent schematically various possible types ofelectromagnetic waves that may be propagated in one form of guide inaccordance with the invention; and

Figs. 12 and 13 illustrate two other embodiments of the invention.

The various types of electromagnetic waves that can be guided through ametallic pipe may be classified as explained in my copendingapplication, supra, into two groups, transverse magnetic (TM) andtransverse electric (TE), the former comprising those waves that arecharacterized by the fact that the vector H representing the magneticcomponent of the field is wholly transverse to the direction ofpropagation and the latter comprising such waves as are characterized inthat it is the electric vector E that lies wholly transverse to thedirection of propagation. Transverse magnetic waves can be described bymeans of characteristic distributions of electrical potential over anyparticular cross-section of the guide and transverse electric waves bymeans of similar distributions of magnetic potential over any particularcrosssection. If a: and 1/ are the Cartesian coordinates of a typicalpoint in a given cross-section, the functions T (:r, y) governing thepotential distribution satisfies the following equation:

=-"x where x is a positive real constant depending upon the size andshape of the pipe and upon the particular characteristic fielddistribution of the wave to be transmitted. For transverse magneticwaves, T is also proportional to the longitudinal electric displacementcurrent density, while for transverse electric waves T is proportionalto the longitudinal magnetic displacement current density.

The various possible characteristic field distributions are in accordwith those solutions of Equation 1 which satisfy appropriate boundaryconditions on the surface of the pipe. Either T or its normal derivativemust vanish on the boundary, depending upon whether the wave istransverse magnetic or transverse electric, re spectively.

Transmission of energy can take place only if the frequency is higherthan a critical frequency depending upon This cut-01f frequency may bederived from the equation w=21rfc, where c is the velocitycharacteristic of light in the dielectric medium within the pipe; thecorresponding wave-length is If the space within the pipe is simplyconnected, X is always greater than zero and there exists a lowerfrequency limit below which there is no transmission mode capable oftransferring energy along the tube.

Equation 1 happens to be also the equation for the displacement of anoscillating elastic membrane, and the different transmission modes forelectromagnetic waves can, with mathematical exactitude, be associatedwith characteristic vibrations of the membrane representing thecrosssection of the pipe. The boundary condition established by thepresence of the metallic pipe in the case of transverse magnetic waves,viz., that longitudinal electric intensity must vanish at the peripheryof the pipe corresponds mathematically with the boundary conditionestablished by fixing the edge of the oscillating membrane. Similarly,in the case of transverse elec tric waves the boundary conditionestablished by the presence of the metallic pipe, viz., thatlongitudinal magnetic intensity must vanish at the periphery of the pipecorresponds mathematically with the boundary condition of a free-edgedoscillating membrane. In brief, the edge of the membrane is to beconsidered fixed or free, depending on whether the corresponding wavesare transverse magnetic or transverse electric. The cut-off frequenciesfor the electromagnetic waves are proportional to the naturalfrequencies of the corresponding membranes. This fact is helpful inobtaining a. qualitative idea of the relationship between variouscut-off frequencies.

It has been observed by Lord Rayleigh with reference to oscillatingmembranes that each new constraint raises the gravest natural frequencyof the membrane and that conversely a removal of a constraint results inlowering the gravest natural frequency. The same observation, applicanthas found, may be made with reference to guided electromagnetic waves.Thus, if a metallic sheet is disposed longitudinally in a tubularmetallic pipe carrying transverse magnetic waves so that it extendsradially inward from the periphery of the pipe, it, as well as the pipeitself, imposes a constraint on the waves, just as clamping a circularmembrane along a radius would impose a constraint on the mode ofvibration of the membrane. Accordingly, the lowest cut-off frequency fortransverse magnetic waves will be raised. On the other hand, if the pipecarries transverse electric waves the radial sheet removes a constraint,just as cutting the corresponding circular membrane from periphery tocenter would do, and both the pipe wall and the radial sheet establishfree-edge boundary conditions. Accordingly, the lowest cut-ofi frequencyfor transverse electric waves will be lowered.

When the metal partition extends from the axis of the tube to itsperiphery, the proper values for x can easily be calculated. Thus inpolar coordinates the potential distribution is given by Likewise, Tmust vanish on the boundary =a of the tube; thus The smallest root ofthis equation corresponds to 11:1; it is The corresponding cut-off isgiven by If the wave is transverse electric, then the normal derivativeof T must vanish on two sides of the partition and on the periphery.Hence, T must be given by the second form in (2) and 0. must be a rootof The smallest root occurs when n=1 and (6) becomes 1 tan Xx J%(x,,)-0or 2 (7) Approximately we have 21rd Xa i In the absence of a partitionthe lowest cut-off frequency for transverse electric Waves in a tubularmetallic guide is that for the so-called TEi,1 (or H1,1) wave, and itcorresponds to ..=1.84, whereas with the partition the correspondingfigure for is 1.16. Thus, the radial partition results in lowering theabsolute cut-off frequency by almost 37 per cent. For transversemagnetic waves, on the other hand, the lowest cut-off frequency in theabsence of a partition corresponds to =2A0 whereas with the partitionthe corresponding figure is 3.14. Thus, for transverse magnetic wavesthe lowest cut-off frequency is raised by nearly 31 per cent. Animportant consequence is that the frequency range between the absolutecut-off frequency and the next higher critical frequency issubstantially widened.

Referring now to Fig. 1 there is shown an embodiment of the presentinvention comprising a metallic tube I, a radially disposed metallicbaflle 2 extending longitudinally through the tube, and the wave source3 connected between the axial edge of the baiile 2 and the oppositepoint of the tube wall. The end of the tube l is closed by a metallicreflector 4 which may be in the form of a copper disc and which isdisposed at such distance from the connecting leads associated with thesource as to obtain maximum transmission of energy along the guide. Theoptimum distance will be found to be approximately a quarter of awave-length or an odd multiple thereof. The source 3 may be consideredas simply a sine wave source but it is representative also of any sourceof intelligence-bearing high frequency waves, such as a carrier wavemodulated with telephone, telegraph, or television signals, or the like.Similarly, at the receiving end of the system, the detector 5 connectedin the same manner as the source 3 is representative of any receivingdevice appropriate for signals of the character generated by the source3. Preferably, the high frequency waves are transmitted in the frequencyrange lying between the absolute cutofl frequency and the next highercut-on frequency to obviate interference from spurious wave types.

Fig. 2 is quite similar to Fig. 1 except that the and connected byparallel leads to the trans versely disposed conductors connecting thebaftie and the lower surface of the pipe.

Fig. 3 is a cross-section of Fig. 1 and it shows the distribution of thetransverse electric field comprising the wave. The designation of thiswave as a Ell wave is consistent with the scheme of designation employedin my copending application, supra.

Fig. 4 shows another type of transverse electric wave that can besustained in the guide of Fig. 1. In its configuration it is quitesimilar to the TEl,l wave, or as it is sometimes known, the H1,1 wave,in an unbafiled tubular metallic guide. The source or receiver may beconnected between the axial edge of the baiile and a point on the pipesuch that the connecting leads are aligned with the electric field, orit may be disposed below the baflie, as illustrated, with connectingleads aligned with the electric field and terminating at the pipe. Asidefrom its other advantages, the battle is useful in this case insuppressing any tendency for the field to rotate out of alignment withthe conductors of the receiving terminal.

Fig. 5 shows still another transverse electric wave, designated as theTEB and a roughly Y-shaped terminal structure conforming with theelectric field and adapted to generate or receive this type of wave.

Transverse magnetic waves of four different types are shown in Figs. 6to 9 as they would appear in the guide of Fig. l. The dotted linesrepresent the lines of magnetomotive force, which lie in transverseplanes.

Figs. 10 and 11 show terminal structures adapted to generate or receivetransverse magnetlc waves of the types illustrated in Figs. 6 and 9,respectively. The structure shown in Fig. 10 comprises two coplanarmetalic plates 6 and I, configured and disposed over the end of themetallic tube so as to leave an elongated gap between them that followsor conforms with the lines of magnetomotive force in the wave. Thetranslating device 3 is electrically connected to the plates 6 and 1 soas, in the case of a generator, to establish an alternating differenceof potential across the gap.

In the terminal structure shown in Fig. 11 there are provided twoapproximately elliptical coplanar electrodes 8 configured and positionedto conform roughly with the magnetic lines depicted in Fig. 9, and thetranslating device 3 connected in operative relation therewith.

In connection with each of the terminal structures herein disclosed, anysuitable means may be employed for purifying the wave generated, thatis, for suppressing spurious wave types that may be producedincidentally to the production of the desired wave type. It iscontemplated too that the relatively ineflicient terminal arrangementsof Figs. 10 and 11 can be improved upon to effect a more efficienttransfer of wave energy between the guide and the source or receiver.

Whereas the radial baflle has in each case been described as extendingfrom the periphery of the guide to its center it will be understood thatthis has been for specific illustrative purposes only and that thebaille may extend a greater or lesser distance as may be desired. Thegreater the radial width of the baflle, the looser is the couplingbetween the two portions of the guide. It will be obvious that thebattle, as the term is used here and in the appended claims, loses itscharacter as such in the limit where it extends completely across theguide, or Where otherwise it serves only to subdivide the guide into aplurality of completely metallically enclosed, electrically independentpassages. Where such independent passages obtain the interior of theguide is multiply connected, whereas the interior is simply connectedif. the battle is radial or if otherwise it is such that atsubstantially every cross-section along the guide there is a (lateral)dielectric connection between all parts of the interior. From theforegoing description of the nature of the invention and from thevarious examples illustrated it is evident that the b ame may be definedgenerally as comprising one or more essentially two-dimensionalconducting strips disposed longitudinally within the guide.

Although the embodiments of the invention hereinbeiore described allutilize a single radial, or generaly transversely disposed planarbaffle, it will be evident from the stated principles underlying theseembodiments that the invention may be embodied in a wide variety ofother forms. Two illustrative modifications are shown in Figs. 12 and13.

Fig. 12 shows a guide comprising an outer cylindrical metallic pipeenclosing three tubular metallic partitions each having a longitudinalopening therein so that at all points along the guide the severalsemi-annular spaces are dielectrically connected. A radial metallicbaffle 2 is provided extending from the periphery of the pipe to theinnermost partition. The translating device 3 is connected by diametralleads from the innermost partition to a point on the outer pipe that isopposite the radial bafiie. The

several partitions and the radial battle are eifective in removingconstraints, for transverse electric Waves, and in permitting operationat lower frequencies. The guide shown in Fig. 13 is provided with aplurality of battles lying in radial planes which also are effective inremoving constraints.

What is claimed is:

1. A transmission system including a wave guide comprising a metallicpipe, means for establishing in said pipe electromagnetic waves of acharacter such that the guide presents to them the characteristics of ahigh-pass filter, and means for depressing the absolute cut-offfrequency of said guide comprising a metallic baflle extendinglongitudinally within said pipe and dividing the interior of. said pipeinto only simply connected portions, said baliie being maintained insubstantially the same relative position throughout the length of saidpipe whereby said waves are unchanged in type.

2. A wave guide comprising a. metallic pipe having a longitudinalconducting ballle therein extending inwardly from the periphery andhaving only one lateral connection with said periphery whereby theinterior of said pipe is a simply connected space, and means forgenerating electromagnetic waves in said guide at a frequency above thecut oif frequency, said bafile being substantially coextensive with saidpipe and the cross-sectional configuration of said pipe and baffle beingsubstantially the same throughout their length.

3. In a signal transmission system, a wave guide comprising a tubularmetallic pipe and a planar longitudinal ballle coextensive with saidpipe and lying in a diametral plane therein, said baille extendinglaterally in one direction only to the periphery of said pipe wherebythe space within said pipe is simply connected, and means fortransmitting through said guide signalmodulated electromagnetic waves atfrequencies above the cut-off frequency of said guide.

4. In a system for the transmission of guided electromagnetic waves of acharacter such that transmission occurs only at frequencies above acritical frequency dependent on the type of wave, a wave guidecomprising a metallic pipe, at least one metallic bafile longitudinallydisposed within said pipe, and means for generating electromagneticwaves for transmission through said pipe, said waves being of such typethat the critical frequency therefor is the absolute cut-off frequencyof said guide and said waves occupying a frequency range lying betweensaid cut-01f frequency and the next higher critical frequency, thecross-sectional configuration of said pipe and bafile beingsubstantially the same throughout the length of said guide whereby saidwaves undergo no change in type, and said baflle being so configured andarranged as to leave a substantially continuous lateral dielectricconnection between all parts of the interior of said pipe.

5. A Wave guide comprising a metallic pipe containing a gaseousdielectric medium and at least one planar metallic balile longitudinallydisposed within said pipe and dividing the interior of said pipethroughout its length into a plurality of symmetrical, laterallypartially bounded compartments, and means for establishingelectromagnetic waves in said guide for transmission therethrough. saidguide presenting to said waves the characteristics of a high-passfilter.

6. A high frequency transmission system including a wave guidecomprising a metallic pipe containing a gaseous dielectric medium and aflat metallic strip disposed longitudinally within said pipe andcoextensive therewith, said strip extending inwardly from the peripheryof said pipe and separating the interior of said pipe into twosymmetrical, laterally dielectrically connected portions, and means forgenerating within said guide, [or transmission therethrough,electromagnetic waves of such characteristic field pattern that they arepropagated only at frequencies exceeding a critical frequency dependenton the transverse dimensions of. said pipe.

7. A combination in accordance with claim 6 in which said generatingmeans comprises a transversely disposed conductor electrically connectedbetween the innermost edge of said strip and a point on said pipe inalignment with said strip, and means interposed in said conductor forapplying an alternating difference of potential thereto.

8. A combination in accordance with claim 6 comprising a translatingdevice within said pipe and electrical connections thereto extendingalong lines of electric force of said waves substantially perpendicularto the plane of said strip.

9. A guide for the transmission of electromagnetic waves of ultra-highfrequency comprising a metallic pipe and a roughly C-shaped channelconcentrically enclosed thereby and substantially coextensive therewithfor reducing the absolute transmission cut-ofi frequency of said guide,said guide having substantially the same cross-sectional configurationthroughout its length.

10. A guide in accordance with claim 9 having in addition a longitudinalplanar baille extending between the walls of said pipe and channel.

11. A uni-conductor guide for ultra-high frequency electromagnetic wavescomprising a tubular metallic pipe having several symmetrically disposedlongitudinal bafiles, said bafiies at every point along said guideextending inwardly and radially from the periphery of said pipe but notto the axis thereof.

12. A wave guide consisting of a metallic pipe having a longitudinalinwardly extending metallic bafile or semi-partition such that the spacewithin said pipe is simply connected and means for applying highfrequency electromagnetic waves to said guide for transmission throughthe interior thereof, said waves being of such character that said guidepresents to them a highpass transmission characteristic and said baffleor semi-partition being so constructed and arranged as to preserveunaltered the characteristic field pattern of said waves.

13. A wave guide comprising a metallic pipe and means for transmittingtherethrough electromagnetic waves or a nature such that the guidepresents to them the characteristics of a high-pass filter, the interiorof said pipe being metallically divided throughout its length into twoor more portions that are continuously and loosely coupled togetherthrough a lateral di electric connection between them, the interior ofsaid pipe being of substantially the same crosssectional configurationthroughout its length whereby the said waves are unchanged in type intheir passage therethrough.

14. An electric signaling system including a wave guiding structurecomprising a tubular metallic pipe, means for launching in said pipe fortransmission therethrough electromagnetic Waves of, a character suchthat the guide presents the characteristics of a high-pass filter, meansfor receiving said waves after transmission through said pipe. saidreceiving means being most effective when disposed in a particularangular relation with reference to the angular orientation of saidwaves, and means for insuring that said particular angular relationobtains comprising means for fixing the angular orientation of saidwaves throughout the length of said guide.

15. A combination in accordance with claim 14 in which thelast-mentioned means comprises a metallic rib on the inner surface ofsaid pipe.

16. A system for the long distance transmission of transverse electricwaves comprising a metallic pipe extending between geographicallyseparated places, said pipe enclosing a simply connected dielectricmedium, and metallic means disposed longitudinally within said pipe andsubstantially coextensive therewith adapted to release a constraint onsaid transverse electric waves, whereby at least one of the criticalfrequencies for said waves is lowered.

SERGEI ALEXANDER SCHELKUNOFF.

